Force on a current currying wire in a magnetic field

[maxelcat]

Homework Statement

This is a multi choice question from last years aqa a level. We know a positive reading is produced on the scales indicating something is pushing down on the scales.
Please see the picture.

Homework Equations

We know current direction, not B field.

The Attempt at a Solution

The positive reading on the scales suggests that the force on the wire is vertically down. Mark scheme says vertically up. We think Newton III is needed but not sure how it works.

Can anyone help explain why i is vertically up.

 

[Doc Al]

The positive reading on the scales suggests that the force on the wire is vertically down.

Is the wire attached to the scales?

 

[maxelcat]

nope. = the wire is not attached to the scales, though it is clamped at points D and E.

 

[Doc Al]

nope. = the wire is not attached to the scales, though it is clamped at points D and E.

Exactly. The yoke is what is attached to the scales. So rethink things with that in mind.

 

[maxelcat]

so the yoke is pushed down. Which means the wire is pushing it down. So there must be a downward force on the wire...

 

[Doc Al]

so the yoke is pushed down.

Yes.

Which means the wire is pushing it down.

Yes.

So there must be a downward force on the wire...

No. Consider Newton's 3rd law.

 

[maxelcat]

well I think you are getting me to say that if the wire is pushing the yoke down then the yoke is pushing it up... but that doesn't make sense to me because I can't see how the interplay between the current and the mag field works. Suppose I then took away the clamps that are holding the wire DE in place and changed nothing else. The wire would move upwards - yes?

 

[Doc Al]

well I think you are getting me to say that if the wire is pushing the yoke down then the yoke is pushing it up...

Sure. Newton's 3rd law in action. If A pushes B up, then B must push A down.

Suppose I then took away the clamps that are holding the wire DE in place and changed nothing else. The wire would move upwards - yes?

Yes.